Cellular radio communication systems are used today in which a plurality of radio communication apparatuses access a base station. In some of such radio communication systems, in order to allow two or more radio communication apparatuses to transmit data pieces to a base station on the same channel at the same time, the data pieces of the two or more radio communication apparatuses are code-division multiplexed by using signal sequences for spreading.
For example, according to a radio communication standard called Long Term Evolution (LTE), pieces of control data of two or more user terminals may be code-division multiplexed on an uplink control channel. An LTE base station uses a Constant Amplitude Zero Auto-Correlation (CAZAC) sequence corresponding to a cell ID. The CAZAC sequence has the property of having an autocorrelation of zero unless the phase difference is zero, and being orthogonal to any of signal sequences obtained by cyclic shifts thereof. On the other hand, the CAZAC sequence has quasi-orthogonality and thus has relatively low cross-correlation with respect to CAZAC sequences other than the signal sequences obtained by cyclic shifts thereof, but does not guarantee perfect orthogonality with respect thereto.
The LTE base station specifies different shift amounts (including zero) to one user terminal and another user terminal belonging to the same cell. Each of the two user terminals cyclically shifts the CAZAC sequence corresponding to the cell ID by the shift amount specified by the base station, and uses the cyclically shifted CAZAC sequence for spread modulation of control data. Thus, an uplink control channel signal transmitted by the one user terminal and an uplink control channel signal transmitted by the other user terminal are orthogonal to each other. Accordingly, the base station is able to separate the two uplink control channel signals even if the two uplink control channel signals are received in a superimposed manner.
As one utilization form of radio communication systems, Machine Type Communication (MTC), also called Machine-to-Machine (M2M) communication, has been discussed. Unlike user terminals such as mobile phones that perform radio communication in response to a user operation, MTC terminals are able to autonomously transmit data without being operated by the user. Examples of MTC terminals include smart meters that measure and report the energy consumption, on-vehicle equipment that monitors and reports the traveling status, electrical household appliances that monitor and report the operating status, and the like. The spread of MTC is likely to greatly increase the number of radio communication apparatuses belonging to each cell.
For supporting a large number of MTC terminals, there has also been a discussion on the method of allocating radio resources to conventional user terminals operated by the users and MTC terminals. For example, in discussions on the LTE radio communication standard, it has been proposed to provide an uplink control channel dedicated to MTC terminals in addition to a conventional uplink control channel used by conventional user terminals.
See, for example, the following documents:
Vodafone, “Proposed SID: Provision of low-cost MTC UEs based on LTE”, RP-111112, 3GPP TSG-RAN meeting #53, September 2011;
Huawei, HiSilicon, CMCC, “Overview on low-cost MTC UEs based on LTE”, R1-112912, 3GPP TSG-RAN WG1 meeting #66bis, October 2011;
Sony Corporation, Sony Europe Limited, “Consideration on Approaches for Low-Cost MTC UEs”, R1-112917, 3GPP TSG-RAN WG1 meeting #66bis, October 2011;
Ericsson, ST-Ericsson, “Standards aspects impacting UE costs”, R1-112929, 3GPP TSG-RAN WG1 meeting #66bis, October 2011; and
IPWireless Inc., “Backwards compatible support for reduced bandwidth MTC LTE UEs”, R1-114268, 3GPP TSG-RAN WG1 meeting #67, November 2011.
Upon assigning signal sequences for code division multiplexing to two or more radio communication apparatuses, the conventional technique described above focuses on the orthogonality between signal sequences in order to prevent interference whenever the radio communication apparatuses transmit data. For example, an LTE base station uses a plurality of orthogonal signal sequences obtained by cyclic shifts of a CAZAC sequence in a cell. However, the number of mutually orthogonal signal sequences is limited. Accordingly, with the conventional technique, there might be a shortage of signal sequences for code division multiplexing if the number of radio communication apparatuses belonging to the cell increases.
When there is a shortage of assignable signal sequences, some of the radio communication apparatuses may be made to wait to transmit data, or the number of channels for transmitting the data may be increased. In the former case, the throughput of the radio communication might be reduced. In the latter case, the usage efficiency of radio resources might be reduced, and scheduling might become complex.
On the other hand, among radio communication apparatuses belonging to the same cell, there may be radio communication apparatuses whose data transmission amount and data transmission frequency are smaller than those of other types of radio communication apparatuses. For example, many MTC terminals are likely to transmit a small amount of data intermittently at long intervals. Transmission signals from such radio communication apparatuses do not frequently interfere with transmission signals from other radio communication apparatuses.